1. Field of the Invention
The present invention relates to a digital video signal recording apparatus and a digital video signal reproducing apparatus for directly recording and reproducing an ATV (Advanced Television) signal to and from a magnetic tape through a rotating head signal, in particular, to an improvement of variable speed reproduction.
2. Description of the Prior Art
Digital VCRs that perform a DCT (Discrete Cosine Transform) operation for an input digital video signal, compress the DCT code into variable length code, and record the compressed code on a magnetic tape through a rotating head are known.
Such digital VCRs have a ST mode (where a video signal corresponding to a conventional television system such as NTSC system is recorded) and a HD mode (where a HDTV signal is recorded). In the ST mode, the video signal is compressed at around 25 Mbps and recorded. In the HD mode, the video signal is compressed at around 50 Mbps and recorded.
As described above, when a HDTV signal is recorded by a digital VCR, the HDTV signal is compressed in the HD mode. However, in the full digital system as with AD-HDTV system, since a signal is compressed and transmitted, the transmitted signal can be directly recorded by a digital VCR. When the received signal is directly recorded by the digital VCR, it is not necessary to decode the HDTV signal from the received signal, compress the HDTV signal, or encode the HDTV signal so as to input the HDTV signal to the digital VCR. Thus, the hardware resource can be reduced.
Such a system has been proposed as an ATV (Advanced Television) system where all processes including a signal process and a signal transmission process are performed with digital signals. As an eminent system of the ATV system, the AD-HDTV system is known. In the AD-HDTV system, a HDTV signal is compressed as a packet corresponding to an image compression technique based on MPEG (Moving Picture Image Coding Experts Group), which is an international standard of moving image.
FIG. 1 is a block diagram showing a construction of a transmission system of the AD-HDTV system. In FIG. 1, reference numeral 101 is a video compression encoder. Reference numeral 102 is an audio encoder. A video signal of the HDTV system is supplied to the video compression encoder 101 through an input terminal 103. An audio signal is supplied to the audio encoder 102 through an input terminal 104.
The video compression encoder 101 compresses the HDTV signal corresponding to a system based on the MPEG system.
In otherwords, the video compression encoder 101 compresses the HDTV signal corresponding to a high efficient encoding system that is a combination of DCT operation and moving compensating operation. As shown in FIG. 2, the video compression encoder 101 outputs an in-frame encoded frame (referred to as an I frame), a forward prediction encoded frame (referred to as a P frame), and a bidirectional prediction encoded frame (referred to as a B code) in a predetermined order. The I frame is independently transformed into a DCT code without the use of the correlation against other frames. On the other hand, the P frame is not directly transformed into a DCT code. Instead, the motion of the P frame is compensated and predicted corresponding to the preceding I frame or the preceding P frame. The difference signal between these frames is transformed into a DCT code. In the case of the B frame, the motion of the B frame is compensated and produced corresponding to the preceding and following I frames or P frames. The difference signal between these frames is transformed into a DCT code. The period in which the I frame takes place is referred to as GOP (Group of Picture). In this example, GOP is 9.
In FIG. 1, reference numeral 105 is a priority encoder. The priority encoder 105 allocates priorities to compressed HDTV signal data. An example of the priorities is as follows.
I frame 1. Frame header 2. Slice header 3. Macro-block address, type, quantizing step 4. DC value 5. Low frequency coefficient 6. High frequency coefficient PA1 P and B frames 1. Frame header 2. Slice header 3. Macro-block address, type, quantizing step 4. Motion vector 5. DC value 6. Low frequency coefficient 7. High frequency coefficient
In the I frame, the frame header has the highest priority. In the order of--slice header,--macro-block address, type, and quantizing step,--DC value,--low frequency coefficient, and--high frequency coefficient, their priorities lower. In the P and B frames, in the order of--frame header,--slice header,--macro-block address, type, and quantizing step,--motion vector,--DC value,--low frequency coefficient, and--high frequency coefficient, their priorities lower.
Reference numeral 106 is a transport encoder. The transport encoder 106 generates a packet corresponding to video data, whose priority is allocated by the priority encoder 105, audio data encoded by the audio encoder 104, and additional information received from an input terminal 107. There are high priority packets and low priority packets. The high priority packets are referred to as HP (High Priority) packets, whereas the low priority packets are referred to as SP (Standard Priority) packets. The ratio between an HP packet and an SP packet is 1 to 4. In a normal image, the frame header, the slice header, the macro-block address, type, and quantizing step, the DC value, and the low frequency coefficient of the I frame are transmitted with an HP packet. On the other hand, the frame header, the slice header, the macro-block address, type and quantizing step, and the motion vector of the P frame and the B frame are transmitted with an HP packet. The HP packet is transmitted with a carrier having a high output power, whereas the SP packet is transmitted with a carrier having a low output power.
FIG. 3 is a schematic diagram showing a construction of a packet. As shown in FIG. 3A, the length of a transmission packet is 148 bytes. A sync is placed at the beginning of the packet. The sync is followed by transmission data and an error correction parity.
FIG. 3B shows transmission data in detail. A service type ST is placed at the beginning of the transmission data. As shown in FIG. 4, the service type ST contains information P, identification information ID, and a counter CC. The information P represents whether the packet is an HP packet or an SP packet. The identification information ID represents transmission data as video data or audio data. The counter CC counts a value from 0 to 15.
The service type ST is followed by an after header AH. FIG. 5A shows an after header of an HP packet. FIG. 5B shows an after header of an SP packet. The after header AH of the HP packet includes a slice start pointer, a frame type, a frame number, a slice number of the frame, and a quantizing factor. The slice start pointer represents the first bit of the input point of transmission data. The after header AH of the SP packet contains a start pointer of a macro-block, a frame type, a frame number, and a macro-block number of the frame.
In FIG. 1, reference numeral 108 is a channel modulator. An HP packet and an SP packet generated by the transport encoder 106 are supplied to a channel modulator 108. The channel modulator 108 modulates the HP packet and the SP packet with two carriers. The output of the channel modulator 108 is supplied to an output terminal 109.
In the AD-HDTV system, by the above-described image compression technique, HDTV signals are transmitted at a transmission rate of 17.4 Mbps. This transmission rate is lower than the record rate in the SD mode of the digital VCR (approximately, 25 Mbps). Thus, signals transmitted in the AD-HDTV system are directly recorded in the SD mode of the digital VCR. When the transmitted signals are directly recorded by the digital VCR, it is not necessary to decode the HDTV signals corresponding to the transmitted signals and input the decoded signals to the digital VCR. Thus, the hardware resource can be reduced. In addition, since the HDTV signals are recorded in the SD mode, the recording time can be prolonged.
However, when the AD-HDTV signals are directly recorded in the SD mode to the digital VCR, the variable speed reproduction cannot be properly performed because of the following reason.
As described above, in the AD-HDTV system, the HDTV signals are compressed corresponding to the MPEG system. In this system, the I frame, which is in-frame encoded, the P frame, which is forward prediction encoded, and the B frame, which is bidirectional prediction encoded, are transmitted. In the variable speed reproduction mode, since the head traverses tracks, continuous frame data cannot be obtained. Thus, data of the P frame and B frame cannot be decoded. Only data of the I frame can be decoded. Data of an I frame is transmitted with an HP packet. Thus, when only data of the I frame is used, the variable speed reproduction can be performed.
When signals corresponding to the AD-HDTV system are directly recorded by the digital VCR, HD packets, which contain the I frames, cannot be satisfactorily obtained in the variable speed reproduction mode. In addition, since the positions of the data of the I frames are not obtained, the data of the I frames equivalent to particular portions of a screen are lost. These portion of the screen may not be updated for a while. Thus, the image quality in the variable speed reproduction mode may be deteriorated.
In such a digital VCR, three types of head constructions are available. In a first construction, two heads with different azimuth angles are disposed in an opposite relation (at an interval of 180.degree.). In a second construction, a double-azimuth head that is constructed of two heads with different azimuth angles is disposed at one position. In a third construction, two double-azimuth heads, each of which is constructed of two heads with different azimuth angles, are disposed in an opposite related (at an interval of 180.degree.). Thus, in the third construction, four heads are used.
As described above, a reproduction valid area which is fixed in the variable speed reproduction mode is used as a surplus record area. HP packet data is recorded in the surplus record area. However, depending on the head construction, some tracks may be not traced by any head. Thus, even if the HP packet data is recorded in the surplus record areas of such tracks, data recorded on these tracks are not reproduced in the variable speed reproduction mode. Thus, since the HP packet data which are recorded on such tracks cannot be reproduced, the HP packet data are lost. Consequently, the reproduced image may be disordered.